Structure of underrun protection

ABSTRACT

A structure of an underrun protection bound to a plurality of brackets disposed at the front or rear portion of a frame of a vehicle is disclosed. The structure has a hollow having a closed section defined by a distal face of a longitudinal direction of the vehicle, a proximal face of the longitudinal direction, an upper face, and a lower face. A plurality of lightening holes are formed in line along an extending direction of the underrun protection on at least one of the distal face and the proximal face. The plurality of lightening holes have diameters in vertical direction set larger as departing farther from binder parts each at which the underrun protection is bound to one of the plurality of brackets.

TECHNICAL FIELD

The present invention relates to the structure of an underrun protectionthat inhibits a vehicle shorter in height from running under anothervehicle longer in height in the event of collision between vehicles.

BACKGROUND

With the intention of inhibiting a vehicle shorter in height such as apassenger car or a mini-car from running under the front or rear portionof a vehicle longer in height such as a motor truck, a vehicle longer inheight equipped with an underrun protection mounted beneath the front orthe rear portion of the frame has been put into practical use.

An underrun protection at least requires enough strength and stiffnessto inhibit another vehicle from running under the vehicle mountingthereon the underrun protection. Unfortunately, an underrun protectionformed of steel, which has been widely used for a vehicle, leads toincrease in weight of the vehicle.

With the above problem in view, an underrun protection has beendeveloped which can ensure requisite strength and stiffness, preventingthe weight from excessively increasing. For example, the followingPatent Literature 1 discloses an underrun protection formed of aluminum.According to the Patent Literature 1, the disclosed underrun protectionsatisfies requisite strength and stiffness, preventing the weight fromexcessively increasing.

PRIOR ART REFERENCE Patent Literature

[Patent Literature 1] Japanese Laid-Open Patent Publication No.2004-175228

SUMMARY Technical Problems

Unfortunately, the presence of an underrun protection, irrespective ofmaterial, leads to increase in total weight of the vehicle and a demandfor lightening as much weight of the underrun protection as possiblearises. One of the solutions of weight reduction of the vehicle is toform one or more lightening holes. In forming such lightening holes,depending on the positions and the ways to make lightening holes, thestrength and the stiffness of the underrun protection may largely lower.

With the foregoing problems in view, the object of the present inventionis to provide a structure of an underrun protection that efficientlyachieves weight reduction by forming lightening holes and also securesrequisite strength and stiffness.

Solution to Problems

(1) In order to attain the above object, there is provided a structureof an underrun protection bound to a plurality of brackets disposed atthe front or rear portion of a frame of a vehicle characterized in that:a hollow having a closed section is defined by a distal face of alongitudinal direction of the vehicle, a proximal face of thelongitudinal direction, an upper face, and a lower face; a plurality oflightening holes are formed in line along an extending direction of theunderrun protection on at least one of the distal face and the proximalface; and the plurality of lightening holes have diameters in verticaldirection set larger as departing farther from binder parts each atwhich the underrun protection is bound to one of the plurality ofbrackets.

(2) The plurality of lightening holes may have diameters in verticaldirection set larger as departing farther from the binder parts, so asto compete against distribution of a bending moment on the underrunprotection, the bending moment being caused by a load input to both edgeportions in the extending direction of the underrun protection from adistal side of the longitudinal direction.

(3) The plurality of lightening holes may comprise at least onerectangular hole having a rectangular front view.

(4) the underrun protection may comprise a distal panel being disposedon a distal side of the longitudinal direction and a proximal panelbeing disposed on a proximal side of the longitudinal direction, thedistal panel and the proximal panel being engaged with each other toform a hollow having a closed section.

(5) The distal face, the proximal face, the upper face, and the lowerface may each take a shape of flat plate; and the closed section of theunderrun protection takes a shape of a rectangle.

(6) The plurality of lightening holes may comprise edge lightening holesbeing formed on the proximal face and being disposed in each edge partoutside the binder parts along the extending direction of the underrunprotection; and the edge lightening holes may have diameters in verticaldirection set larger as departing farther from the binder parts towardthe outer side in the extending direction of the underrun protection.

(7) The underrun protection may comprise a distal panel having achannel-shaped section and an proximal panel having a channel-shaped ora straight line section bound together to form a hollow having a closedsection; and the distal face may be formed of a web of the distal panel,the proximal face may be formed of a web of the proximal panel, and theupper face and the lower face may each be formed of flanges of thedistal panel or the proximal panel.

(8) The distal panel may comprise edge bents formed by bending thedistal panel toward the proximal panel at both edge portions in theextending direction; and the proximal panel may be formed to have auniform cross section along the extending direction.

Effects of Invention

The structure of the underrun protection of the present invention formsmultiple lightening holes being arranged in the extending direction ofthe underrun protection on the distal face or the proximal face andhaving larger vertical hole diameter as departing farther from thebinder parts where the underrun protection is bound to the brackets.This configuration effectively enhances the weight reduction of theunderrun protection, securing the requisite strength and stiffness.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view showing an underrun protection according toa first embodiment of the present invention and the main part of theperipheral structure of the underrun protection;

FIG. 2 is a top view of an underrun protection of the first embodiment,overlapping the largeness of the bending moment acting on the underrunprotection;

FIG. 3 is a perspective view solely showing a distal panel of anunderrun protection of the first embodiment;

FIG. 4 is a top view solely showing a distal panel of an underrunprotection of the first embodiment;

FIG. 5 is a perspective view solely showing a proximal panel of anunderrun protection of the first embodiment;

FIG. 6 is sectional views of an underrun protection of the firstembodiment, FIG. 6A being a cross sectional view of a binder part andFIG. 6B being a cross sectional view of a middle part or an edge part;

FIG. 7 is a front view of an underrun protection of the firstembodiment, removing a distal panel, also showing a bending momentacting on the underrun protection;

FIG. 8 is a front view illustrating the main part of an underrunprotection of the first embodiment removing distal panel, enlarging theedge in the width direction and the peripheral part thereof;

FIG. 9 is a front view illustrating the main part of an underrunprotection of the first embodiment removing distal panel, enlarging thecenter in the width direction and the peripheral part thereof; and

FIG. 10 is a perspective view solely showing a reinforcing panel of anunderrun protection of the first embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, description will now be made in relation to embodiments ofthe present invention with reference to the accompanying drawings.

An underrun protection of the present invention is mounted on the frontor rear portion of a vehicle such as a motor truck (hereinafter simplycalled “vehicle”),

An underrun protection mounted on the front portion of a vehicle is alsocalled “front underrun protection” or its abbreviation “FUP”, andinhibits another vehicle short in height from running under the frontportion of the vehicle equipped with the FUP. Conversely, an underrunprotection mounted on the rear portion of a vehicle is also called “rearunderrun protection” or its abbreviation “RUP”, and inhibits anothervehicle short in height from running under the rear portion of thevehicle equipped with the RUP.

A normal front underrun protection is formed to have a shape at the endsin the width direction of the vehicle along with the bumper.Specifically, a FUP has a shape of slightly bending backward asapproaching the ends in the width direction. In contrast, a normal rearunderrun protection is formed to have a straight line in the widthdirection of the vehicle.

The description below assumes that the direction that the vehicle runsis referred to as the “forward” direction; the “left” and the “right” isbased on the forward direction; the opposite direction of the forwarddirection is referred to as the “backward” or “rearward”; “downward” isa direction that the gravity acts; “upward” is the opposite direction tothe downward; and a “width direction” is a direction that isperpendicular to both the longitudinal direction and the verticaldirection.

A “proximal side” of the vehicle represents a position closer to thecenter of the vehicle; and a “distal side” represents a positionopposite to the proximal side with respect to the center of the vehicle.Accordingly, for a front underrun protection that is mounted to thefront portion of a vehicle, the proximal side of the longitudinaldirection of the vehicle means a rear portion of the FUP, and the distalside of the longitudinal direction of the vehicle means a front portionof the FUP. For a rear underrun protection that is connected to the rearportion of a vehicle, the proximal side of the longitudinal direction ofthe vehicle means a front portion of the RUP and the distal side of thelongitudinal direction means a rear portion of the RUP.

The wording “inside” and “outside” of an underrun protection simplyrefer to the inside of a hollow space and the outside of the hollowspace, respectively.

Hereinafter, the description to be made assumes that the underrunprotection is a front underrun protection mounted to the front portionof the vehicle frame, and the front underrun protection is simplyreferred to as the “underrun protection”.

First Embodiment

[1. Structure]

Firstly, description will now be made in relation to a peripheralstructure of an underrun protection according to the first embodiment.

As shown in FIG. 1, an underrun protection 10 is connected to the lowerportion of a front portion 1 a of the frame 1 such that the longitudinalside of the underrun protection 10 extends along the width direction ofthe vehicle. The underrun protection 10 is connected to brackets 5, 5that are respectively formed on the left and right portions of the frontportion 1 a of the frame 1.

[1-1. Peripheral Structure]

Hereinafter, description will now be made in relation to the structureof the frame 1 and the bracket 5 in sequence.

[1-1-1. Frame]

The frame 1 of the vehicle is a so-called ladder frame, and includes apair of side frames 2, 2 and a cross member 3.

The side frames 2, 2 are arranged along the longitudinal direction ofthe vehicle to have a predetermined distance in the width directiontherebetween. The side frames 2, 2 are disposed inside of both ends inthe width to have a distance shorter than overall width of the vehicleand, wheels and side-body parts, which are not shown in the drawings,are arranged at the outside of the respective side frames 2, 2 in thewidth direction. The shape and arrangement of side frames 2, 2 aresymmetry on the basis of the center line of the width direction. For theabove, description to be hereinafter made will focus on one (left) ofthe side frames 2, 2 and description of the other side frame 2 will beomitted.

Each side frame 2 has a C-shaped, i.e., channel-shaped, vertical sectionalong the width direction. The side frame 2 has a web 2 a verticallyarranged and flanges 2 b, 2 b that are bent so as to protrude from theupper and lower ends of the web 1 inwardly along the width direction ofthe vehicle.

The cross member 3 is arranged along the width direction of the vehicle.Although only one cross member 3 disposed at the front portion 1 a ofthe frame 1 appears in FIG. 1, non-illustrated multiple cross membersare disposed along the longitudinal direction at predeterminedintervals.

The cross member 3 is formed to have a C-shaped, i.e., channel-shaped,vertical section along the longitudinal direction. The cross member 3has a web 3 a vertically arranged and flanges 3 b, 3 b formed by bendingthe upper and lower edges of the web 3 a in the same direction. In theillustrated example, the flanges 3 b, 3 b are formed by bending theedges so as to protrude rearward of the vehicle.

The cross member 3 is bound to the side frames 2, 2 at the both ends ofthe width direction, so that the cross member 3 connects one side frame2 to the other side frame 2. Alternatively, a non-illustrated gusset maybe interposed between the cross member 3 and each side frame 2, and thecross member 3 and the side frame 2 are engaged with the gusset, so thatthe cross member 3 may be bound to the two side frames 2, 2 via suchgussets. Further alternatively, non-illustrated faces that overlap webs2 a of the respective side frames 2, 2 may be formed on the both ends ofthe cross member 3 in the width direction of the vehicle, and the facesof the cross member 3 may be engaged with the webs 2 a of the respectiveside frames 2, 2.

Various driving components, such as an engine and a transmission, a cab,a carrier, and various accessories, all of which do not appear in thedrawings, are installed to and supported by the frame 1 which iscollectively configured by the side frames 2, 2 and the cross member 3.For the purpose of installing heavy components, the side frames 2, 2 andthe cross member 3 have sufficient strength and stiffness.

[1-1-2. Brackets]

A pair of brackets 5, 5 are disposed at the left and right portions toinstall the underrun protection 10 to the frame 1, and are specificallybound to the front portion is of the frame 1. In detail, the leftbracket 5 is installed to the front portion of the side frame 2 disposedon left while the right bracket 5 is installed to the front portion ofthe side frame 2 disposed on right. The underrun protection 10 issupported at the portions (in the illustrated example, two portions)corresponding to the brackets 5, 5 having a predetermined distance inthe width direction of the vehicle. The brackets 5, 5 have the symmetricarrangement and the symmetric shape with reference to the center of thewidth direction.

The brackets 5, 5 are arranged downward of the frame 1 (in this example,the side frames 2, 2) so that the underrun protection 10 is arranged atthe lower position than the frame 1. Since the underrun protection 10needs to endure load applied backward to the front of the vehicle, theunderrun protection 10 is bound to the front portions of the brackets 5,5, which thereby support the underrun protection 10 from behind.

Various structures can be applied to the brackets 5, 5, but the firstembodiment selects the following structure for the brackets 5, 5. Here,description will now be made, focusing on one of the brackets 5, 5(i.e., the left bracket 5 in FIG. 1).

The bracket 5 includes a first panel 6, a second panel 7, and a thirdpanel 8 from the inside to the outside along the width direction of thevehicle.

The first panel 6 is arranged along the outer face of the web 2 a, whichis included in the corresponding side frame 2 and which is extendingalong the vertical direction and the longitudinal direction of thevehicle. The first panel 6 is formed of a main body 6 a in the form of aflat plate to be engaged with the web 2 a of the side frame 2, areinforcing flange 6 b formed by bending the front edge of the main body6 a, and another reinforcing flange 6 c formed by bending the rear edgeof the main body 6 a.

The upper portion of the main body 6 a of the first panel 6 is engagedwith the web 2 a of the side frame 2 while the lower portion of the mainbody 6 a is overlapped and engaged with the second panel 7 and the thirdpanel 8, keeping in contact with one another. In this example, thefront-edge reinforcing flange 6 b is formed by bending the front portionof the main body 6 a inwardly along the width direction of the vehicleand the rear-edge reinforcing flange 6 c is formed by bending the rearportion of the main body 6 a from the upper end to the lower end of themain body 6 a outwardly along the width direction of the vehicle.

The second panel 7 and the third panel 8 respectively include mainbodies 7 a and 8 a, which are, in this sequence, to overlap and beengaged with the lower portion of the outer face of the main body 6 a inthe order. An installation portion 7 b is formed at the front edge ofthe second panel 7 and includes an installation face 9 a extendingtoward the inside of the width direction of the vehicle and along thelateral and the vertical directions. Likewise, an installation portion 8b is formed at the front edge of the third panel 8 and includes aninstallation face 9 b extending toward the outside of the widthdirection of the vehicle and along the lateral and the verticaldirections.

The installation faces 9 a and 9 b (hereinafter collectively referred toas “installation face 9”) of the installation portion 7 b and 8 b areflush with each other, and are overlapped with and bound to the backface (corresponding to a rear face 10 b of the main body 10A of underrunprotection 10 that will be detailed below) of the underrun protection10.

At the rear edges of the second panel 7 and the third panel 8,reinforcing flanges 7 c and 8 c are formed by bending the correspondingportions outwardly along the width direction of the vehicle.

The respective components and parts, such as the side frames 2, thecross member 3, and the brackets 5 can be bound to each other by variousknown method of binding, such as welding bind including plug welding andfillet welding, or fastening with rivets, and bolts and nuts.

[1-2. Underrun Protection]

Next, description will now be made in relation to the structure of theunderrun protection 10.

The underrun protection 10 is arranged to across the entirety of thewidth direction of the vehicle. As illustrated in FIGS. 1 and 2, theunderrun protection 10 has parts 11, 11 (hereinafter referred to as“binder parts 11, 11) between the middle part of a extending directionof the underrun protection 10 (i.e., the width direction of the vehicle)and each edge, which parts are bound to the respective brackets 5, 5(only the second and third panels 7 and 8 appear in FIG. 2, which omitsillustration of the first panel 6). In addition to the two binder parts11, 11, the underrun protection 10 is divided into multiple parts of:edge parts 12 outside the respective binder parts 11, 11 in the widthdirection of the vehicle; and a middle part 13 between the binder parts11, 11 and inside the binder parts 11, 11 in the width direction of thevehicle.

Namely, the underrun protection 10 is formed of five parts, arrangedfrom one end to the other end of the lateral direction, in the order ofthe edge part 12, the binder part 11, the middle part 13, the binderpart 11, and the edge part 12.

The binder parts 11, 11 are defined outside the middle part 13 in thewidth direction of the vehicle and are bound to the respective brackets5, 5, so that the middle part 13 has a structure of a both-end-supportedbeam. In contrast, the binder parts 11, 11 are defined at the inside ofthe respective edge parts 12, 12, so that the edge parts 12 each have astructure of cantilevered.

For example, loads P1, P2, and P3 shown in FIG. 2 that have been taughtto be used in tests for security standard can be applied to the standardmeasurement of the strength and the stiffness that the underrunprotection 10 requires. Specifically, the loads P1, P2, and P3 are inputinto the underrun protection 10, and if the underrun protection 10successfully endures the loads P1, P2, and P3, the underrun protection10 satisfies the required strength and stiffness.

The loads P1, P2, and P3 are input into the underrun protection 10 atthe respective corresponding positions in the width direction of thevehicle as illustrated in FIG. 2 from the ahead of the underrunprotection 10 in the longitudinal direction and the horizontaldirection.

The load P1 is input into each edge part 12. Here, the load P1 isassumed to be input at a point which is inwardly distant from the end ofthe underrun protection 10 by a predetermined length within each edgepart 12.

The load P2 is input into each binder part 11. Here, the load P2 isassumed to be input into an installing point S on the underrunprotection 10 where the corresponding bracket 5 is connected. In theillustrated example, the installing point S corresponds to an engagingface at which the first panel 6 (see FIG. 1) is engaged with the secondpanel 7 in the bracket 5, that is, the center (face center) of theinstallation face 9 of the bracket 5 along the width direction of thevehicle.

The load P3 is input to the center of the middle part 13. Specifically,the load P3 is input to the middle part 13 at the center along thelateral direction, i.e., at the center C of the underrun protection 10along the width direction of the vehicle.

The loads P1, P2 and P3 can be determined in accordance with the legalsafety standard of each country. If these loads P1, P2 and P3 areplanarly input, the face centers can be regarded as the respective inputpoint.

As the above, the load P2 is input into each binder part 11, which issupported by the corresponding bracket 5, while the loads P1 and P3 areinput into the edge parts 12 and the middle part 13, which have beamstructures based on the bracket 5. Thereby, a bending moment caused fromthe loads P1 and P3 acts on the underrun protection 10.

The largeness of the bending moment depends not only on the loads to beinput but also on the supporting manner of the beam structures, thedistances from the input points (where force is applied) of the loads,and the distance from fulcrums of the beam structures (in thisembodiment, corresponding to the distances from the installing points Sof the respective brackets 5). Since each load P1 is applied to the freeend of the cantilever, a remarkably large bending moment acts on thefulcrum (i.e., each binder part 11). Meanwhile, since the load P3 isapplied to the center of the both-end-supported beam structure, thebending moment acting on the input point of the load P3 is opposite indirection to the bending moments acting on the fulcrum of the load P3 sothat the bending moment at the input point and the both fulcrums arekept to be lower than that acting on the fulcrum by the load P1.

Accordingly, it is important for the underrun protection 10 to competeagainst the bending moment caused by the load P1. Therefore, descriptionwill now focus on the bending moment caused by the load P1 input intothe edge part 12 having a cantilever structure.

As illustrated by the thick solid lines in FIG. 2, input of the load P1into the left edge part 12 (i.e., the edge part 12 on the left in thewidth direction of the vehicle, hereinafter simply called “left” edgepart 12) generates a bending moment having a largeness depending on thedistance between the installing point S of the bracket 5 to the leftbinder part 11 and the input point of the load P1.

Accordingly, when the load P1 is input into the left edge part 12, thelargest bending moment acts on the installing point S of the leftbracket 5. In other words, the largeness of the bending moment resultingfrom input of the load P1 linearly increases from the input point of theP1 to the installing point S of the left bracket 5. This means that arequired second moment of area in the edge part 12 (here, the left edgepart 12) increases as inwardly approaching the underrun protection 10along the width direction of the vehicle.

Likewise, as illustrated by the thick broken line in FIG. 2, thelargeness of the bending moment resulting from input of the load P1input into the other edge part 12 on the right of the vehicle(hereinafter simply called the “right” edge part 12) linearly increasesfrom the input point of the P1 to the installing point S of the rightbracket 5.

Since the middle part 13 of the underrun protection 10 has a structureof a both-end-supported beam in which the binder parts 11, 11 are boundto and supported by the respective brackets 5, 5, the largeness of thebending moment caused by the load P1 input into the left edge part 12linearly decreases in the direction from the installing point S of theleft bracket 5 to the installing point S of the right bracket 5, asshown by the thick solid line in FIG. 2. In contrast, as illustrated bythe thick broken line in FIG. 2, the largeness of the bending momentcaused by the load P1 input into the right edge part 12 linearlydecreases within the middle part 13 in the direction from the installingpoint S of the right bracket 5 to the installing point S of the leftbracket 5.

In the event of offset collision of the vehicle equipped with theunderrun protection 10 with a vehicle short in height, load is inputinto either one of the left and right edges of the underrun protection10. Accordingly, the underrun protection 10 requires a stiffness thatcan compete a load input into either one of the left side and the rightside of the vehicle. In other words, the middle part 13 of the underrunprotection 10 requires a stiffness that can compete a larger one of thebending moment caused by inputting the load P1 into the left edge part12 and the bending moment caused by inputting the load P1 into the rightedge part 12. In the middle part 13 of the underrun protection 10, thelargeness of a required second moment of area decreases in the directionfrom the installing point S of each bracket 5 to the center C of thewidth direction of the vehicle.

Hereinafter, description will now be made in relation to each componentof the underrun protection 10.

Since the underrun protection 10 here is a front underrun protection,the distal side of the longitudinal direction of the vehicle is front orforward side of the vehicle while the proximal side of the longitudinaldirection of the vehicle is a rear side or rearward of the vehicle.

As illustrated in FIG. 1, the underrun protection 10 includes a mainbody 10A, and reinforcing panels 40, 40 (both shown by broken lines)that reinforce the main body 10A. At the both ends along the extendingdirection of the underrun protection 10, non-illustrated caps may bearranged to cover the both ends.

The main body 10A includes a front face (distal face along thelongitudinal direction of the vehicle) 10 a, a rear face (proximal facealong the longitudinal direction of the vehicle) 10 b, an upper face 10c, and a lower face portion 10 d. These four faces 10 a, 10 b, 10 c, and10 d collectively form a hollow space. In other words, the main body 10Aforms the hollow having a rectangular closed section defined by the fourfaces 10 a, 10 b, 10 c, and 10 d. The faces 10 a, 10 b, 10 c, and 10 dof the main body 10A correspond to the respective faces of the underrunprotection 10.

The main body 10A extends along the extending direction of the underrunprotection 10, and is formed of a distal panel 20 and a proximal panel30 arranged. This means that the main body 10A is formed by binding thedistal panel 20 arranged at the distal side of the vehicle with theproximal panel 30 arranged at the proximal side of the vehicle.

Reinforcing panels 40, 40 reinforce the main body 10A. The reinforcingpanels 40, 40 are symmetrically arranged and shaped along the widthdirection of the vehicle with respect to the center line of the widthdirection. For the above, the description that would be hereinafter madefocuses on one (left) reinforcing panel 40, and description of the other40 will be omitted here.

Focusing on the extending direction of the underrun protection 10, thereinforcing panel 40 is arranged so as to extend from the edge part 12on the side of the binder part 11 to the middle part 13 on the side ofthe same binder part 11 as to be detailed below. Namely, the reinforcingpanel 40 overlaps the entirety of the binder part 11 and has the bothends overlapping portions of the edge part 12 and the middle part 13.

The reinforcing panel 40 is arranged so as to be in contact with theinner face of the rear face 10 b inside the hollow of the main body 10A.

In the underrun protection 10, a multiplexed panel part 80 (partiallyappearing in FIG. 1) represents a part having a cross section being apart (see, FIG. 6, hereinafter called a “reinforcing overlapping part”)81 at which the main body 10A is in contact with the reinforcing panel40 along the extending direction of the underrun protection 10 (i.e.,the width direction of the vehicle). In contrast, a part at which thereinforcing panel 40 is not in contact with the main body 10A and whichtherefore has a cross section formed only of the main body 10A becausethe reinforcing panel 40 does not overlap is called simplex body part 90(partially appearing in FIG. 1).

These panels 20, 30, and 40 are made of ultrahigh tensile strength steelplates, which are defined to have a tensile strength of 980 MPa or morein the illustrated example. Examples of the ultrahigh tensile strengthsteel include a solute strengthening steel plate or a precipitationhardening stainless steel plate strengthened by adding an element suchas nickel (Ni), silicon (Si), or manganese (Mn) along with carbon (C)into steel material, and a dual phase steel plate strengthened byquenching after stamping. The commodity products of ultrahigh tensilestrength steel plates have a tensile strength of 980 MPa, 1.2 GPa, or1.5 GPa.

In this example, the panels 20, 30, and 40 are formed of so-called 980type (having a tensile strength of 980 MPa). An ultrahigh tensilestrength steel plate having a thickness of 3.2 mm can be used here. Thismeans that the panels 20, 30, and 40 can be formed of the same ultrahightensile strength steel plates (the same material) having the samethickness.

The panels 20, 30, and 40 constituting the underrun protection 10 may beformed by stamping.

Hereinafter, description will now be made in relation to, in sequence,the distal panel 20, the proximal panel 30, and the reinforcing panel40.

[1-2-1. Distal Panel]

As shown in FIG. 3, the distal panel 20 is formed to have a C-shapedsection, that is, a channel-shaped section (i.e., the cross sectionorthogonal to the extending direction of the underrun protection 10).Accordingly, the distal panel 20 includes a vertical web 21 and flanges22 and 23 formed by bending the upper edge and the lower edge of the web21 in the same direction so as to protrude rearward in the vehicle.Namely, the distal panel 20 has the web 21 arranged at the distal sideof the vehicle and the flanges 22 and 23 arranged to protrude rearwardin the vehicle. Besides, the distal panel 20 includes edge bents 29formed at the both edge portions of the extending direction of the panel20 by bending the edge portions toward the proximal panel 30 (see FIG.1). In FIG. 3, the reference number is attached to only one of the edgebents 29.

The web 21 of the distal panel 20 extends along the width direction ofthe vehicle and is arranged vertically.

The flanges 22 and 23 extend along the width direction and are arrangedhorizontally. The structure of the flanges 22 and 23 are the same exceptthat the flange 22 is connected to the upper edge of the web 21 whilethe flange 23 is connected to the lower edge of the web 21. Consideringabove, the one flange 22 (the upper one of the flanges 22 and 23) willnow be detailed.

As illustrated in FIG. 4, multiple lightening holes 51, 52, and 53 areformed on the flange 22 of the distal panel 20. Likewise, the lighteningholes are formed on the other flange 23 (see FIG. 3) at thecorresponding points as those on the flange 22. Namely, the flange 22has the lightening holes at the same horizontal positions as thoseformed on the other flange 23.

The flanges 22 and 23 of the distal panel 20 each extend to a positionincluding a lightening hole forming part 96 that covers a middle portionof the underrun protection 10 along the longitudinal direction of thevehicle and that includes an equidistance portion 95 having the samedistance from the front face 10 a and the rear face 10 b of the underrunprotection 10. The equidistance portion 95 and the lightening holeforming part 96 are shown in FIG. 6B.

Among the multiple lightening holes 51, 52, and 53, the middlelightening holes 51 and 52 (in FIG. 4, reference numbers 51 and 52 areeach attached to one of the holes) formed in the middle part 13 arediscriminated from the edge lightening hole 53 formed in edge part 12.Furthermore, the middle lightening holes 51 and 52 can be classifiedinto lightening holes (hereinafter called circular middle lighteningholes) 51 each having a circular opening when seen from the top andlightening holes (hereinafter called oval middle lightening holes) 52each having an oval opening when seen from the top. The oval middlelightening holes 52 shown in FIG. 4 are each defined by two parallellines extending along the lateral direction of the distal panel 20 andarcs that connects the front ends of the two lines with the rear ends ofthe two lines, but alternatively, each oval middle lightening hole 52may have an ellipse shape having the major axis along the extendingdirection of the underrun protection 10.

Each circular middle lightening hole 51 has a diameter of the extendingdirection of the underrun protection 10 different from a diameter ofoval middle lightening hole 52 of the lateral direction, but thediameter of the minor axis (along the longitudinal direction of thevehicle) of the oval middle lightening holes 52 are set to be the sameas the diameters of the circular middle lightening holes 51. The middlelightening holes 51 and 52 are formed to be horizontally symmetric.

In FIG. 4, the center of the extending direction of the underrunprotection 10 (i.e., the center C of the width direction of the vehicle)and one of circular middle lightening holes 51 are concentricallyarranged, and in the direction from the concentric circular middlelightening hole 51 to the both ends of the width direction of thevehicle, an oval middle lightening hole 52, a circular middle lighteninghole 51, an oval middle lightening hole 52, and an oval middlelightening hole 52 are arranged in this order. However, arrangement ofthe circular middle lightening holes 51 and the oval middle lighteningholes 52 are not limited to the above, and various arrangements can beadopted.

The middle lightening holes 51 and 52 may be arranged at constantintervals or may be arranged such that the intervals come to be longeras approaching, from the center C, the ends along the width direction ofthe vehicle.

Furthermore, the circles, the ovals, and the ellipses of the middlelightening holes 51 and 52 are examples of the top views thereof andalternatively the middle lightening holes 51 and 52 may have arbitrarytop views. For example, the middle lightening holes may be set to havedifferent diameters of the extending direction of the underrunprotection 10 that come to be large as approaching, from the center C,the ends along the width direction of the vehicle. Also in this case,the middle lightening holes may be arranged at constant intervals or maybe arranged such that the intervals come to be longer as approaching,from the center C, the ends of the width direction of the vehicle.

In contrast, all the middle lightening holes may be formed to have thesame shape. Also in this case, the middle lightening holes may bearranged at constant intervals or may be arranged such that theintervals come to be longer as approaching, from the center C, the endsof the width direction of the vehicle.

FIG. 4 illustrates each binder part 11 forming thereon two holes, whichare to be used for plug welding to engage the distal panel 20 with theproximal panel 30.

Each edge lightening hole 53 of the first embodiment is formed to have acircular top view as with the circular middle lightening hole 51. Theedge lightening hole 53 is formed within the edge part 12 and is invicinity of the binder part 11. The edge lightening hole 53 may have adifferent top view from a circle and may be omitted to simplify thestructure.

[1-2-2. Proximal Panel]

As shown in FIG. 5, the proximal panel 30 is formed to have a C-shapedsection, that is, a channel-shaped section (i.e., the cross sectionorthogonal to the extending direction of the underrun protection 10).Accordingly, the proximal panel 30 includes a vertically-arranged web 31and flanges 32 and 33 formed by bending the upper edge and the loweredge of the web 31 in the same direction so as to protrude forward inthe vehicle. Namely, the proximal panel 30 has the web 31 arranged atthe proximal side of the vehicle and the edges of the flanges 32 and 33arranged to protrude forward in the vehicle. The proximal panel 30 isformed to have a uniform cross section along the extending direction ofthe underrun protection 10.

The web 31 extends along the width direction of the vehicle and isarranged vertically.

The flanges 32 and 33 extend along the width direction and are arrangedhorizontally. The structure of the flanges 32 and 33 is the same exceptthat the flange 32 is connected to the upper edge of the web 31 whilethe flange 33 is connected to the lower edge of the web 31.

As shown in FIGS. 5 and 7, multiple holes 61-65 are formed on the web 31of the proximal panel 30 along the extending direction of the underrunprotection 10. The multiple holes 61-65 are classified into lighteningholes 61-64 for lightening the underrun protection 10 and binding holes65 that are used for binding elements through fastening. The referencenumber 65 indicating a binding hole is applied to a single binding holein FIG. 7. The lightening holes 61 and 62 are, as shown in FIG. 7,formed on the simplex body part 90 (partially appears in FIG. 7).Multiple edge lightening holes 61 are formed on the web 31 of theproximal panel 30 in the respective edge parts 12 and multiple middlelightening holes 62 are formed on the web 31 of the proximal panel 30 inthe middle part 13. As shown in FIG. 7, the lightening holes 63 and 64and the binding holes 65 (see FIG. 5) are formed on the multiplexedpanel part 80 (partially appears in FIG. 7).

First of all, description will now be made in relation to the edgelightening holes 61 formed on the simplex body part 90.

As illustrated in FIG. 7, multiple (three in this example) edgelightening holes 61 are formed in each edge part 12. Here, the threeedge lightening holes 61 on the edge part 12 are referred to as a firstedge lightening hole 61 a, a second edge lightening hole 61 b, and athird edge lightening hole 61 c that are sequentially arranged from theinside to the outside along the width direction of the vehicle. The edgelightening holes 61 a, 61 b, and 61 c are arranged on the simplex bodypart 90 outside the reinforcing panel 40 in the width direction of thevehicle and have respective centers positioned at the center of the web31 of the proximal panel 30 along the vertical direction.

FIG. 8 is an enlarged view of Part A of FIG. 7 and specifically showsthe edge and the peripheral of the edge of the underrun protection 10along the width direction of the vehicle from which the distal panel 20is detached. As shown in FIG. 8, among edge lightening holes 61 formedon the simplex body part 90 in one (here left) edge part 12, the firstedge lightening hole 61 a is a circular hole having a circular frontview, the second edge lightening hole 61 b and the third edge lighteninghole 61 c are both rectangular holes each having a rectangular frontview. Bolts for connecting a non-illustrated bumper of the cab isarranged between the second and third edge lightening holes 61 b and 61c.

The edge lightening holes 61 are set to have larger hole diameters alongthe vertical direction as departing from the respective binder parts 11.In this example, the hole diameters along the vertical direction of theedge lightening holes 61 come to be larger in the order of the firstedge lightening hole 61 a, the second edge lightening hole 61 b, and thethird edge lightening hole 61 c.

The hole diameters along the vertical direction of the edge lighteningholes 61 are set to be able to compete against the distribution of thebending moment being caused by the load P1 on the underrun protection10. As detailed above, the largeness of the bending moment being causedby the load P1 and acting on the underrun protection 10 linearlydecreases in the direction from the fulcrum (here, the installing pointS of the corresponding bracket 5) at the inner part along the widthdirection to the input point of the load P1 (see FIGS. 2 and 7). For theabove, the underrun protection 10 requires bending stiffness thatlinearly decreases as coming from the installing point S, serving as afulcrum, closer to the input point of the load P1.

In order to satisfy the linear change of the required bending stiffness,the edge lightening holes 61 a, 61 b, and 61 c are set to have therespective hole diameters d₁, d₂, and d₃ of the vertical direction thatsubstantially linearly increase as approaching, from the inner side ofthe vehicle, the input point of the load P1. The hole diameters d₁, d₂,and d₃ here correspond to the vertical diameters of the edge lighteningholes 61 a-61 c at centers of the respective holes of the widthdirection. Accordingly, the lines connecting the top ends and the bottomends of the holes defining the respective hole diameters in the verticaldirection are substantial straight lines as one-dot broken lines show inFIG. 8. The second moment of area at a portion where the edge lighteningholes 61 a, 61 b, and 61 c are formed of in each edge part 12 of theunderrun protection 10 at which parts decreases as approaching, from theinner side of the vehicle, the input point of the load P1, and satisfiesthe requisite bending stiffness.

Next, description will now be made in relation to the middle lighteningholes 62 formed on the simplex body part 90.

As illustrated in FIG. 7, multiple (three in this example) middlelightening holes 62 are formed in the middle part 13. In this example,the three middle lightening holes 62 are referred to as a first middlelightening hole 62 a, a second middle lightening hole 62 b, and a thirdmiddle lightening hole 62 c from the left to the right along the widthdirection of the vehicle. These middle lightening holes 62 a, 62 b, and62 c are horizontally arranged along the lateral direction at the middleportion between the binder parts 11, 11 and are arranged on the simplexbody part 90 arranged at an inner part of the two reinforcing panels 40,40 such that the centers of the respective holes 62 a-62 c are arrangedat the vertical middle points of the web 31 of the proximal panel 30.

FIG. 9 is an enlarged view of the part B of FIG. 7, illustrating thecenter of the underrun protection 10 and the peripheral part of thecenter along the width direction of the vehicle from which the distalpanel 20 is removed. As illustrated in FIG. 9, the second middlelightening hole 62 b is a rectangular hole having a rectangular frontview and is arranged at the center C of the width direction of thevehicle. The first middle lightening hole 62 a and the third middlelightening hole 62 c are circular holes each having a circle front view.The opening area of the first middle lightening hole 62 a is the same asthat of the third middle lightening hole 62 c and the both holes 62 aand 62 c are equally distant from the first (sic: second) middlelightening hole 62 b.

The middle lightening holes 62 are set to have larger hole diameter ofthe vertical direction as departing farther from the binder parts 11.This means that the second middle lightening hole 62 b disposed at thecenters of the width direction of the vehicle has a smaller (sic:larger) hole diameter in the vertical direction than those of the firstmiddle lightening hole 62 a and the third middle lightening hole 62 badjacent to the binder parts 11, 11.

The hole diameters of the middle lightening holes 62 in the verticaldirection are set so as to compete against the distribution of thebending moment caused by the load P1 on the underrun protection 10. Asdescribed above with reference to FIGS. 2 and 7, the largeness of thebending moment that is caused by the load P1 and that acts on theunderrun protection 10 linearly decreases in the middle part 13 asapproaching the center C of the width direction of the vehicle from thefulcrum (here, the installing point S of the bracket 5) at the innerside of the vehicle. For the above, the underrun protection 10 requiresbending stiffness that linearly decreases as approaching the center C ofthe width direction from the installing point S, serving as a fulcrum.

In order to satisfy the linear change of the required bending stiffness,the middle lightening holes 62 a, 62 b, and 62 c are set to have therespective hole diameters d₄, d₅, and d₆ of the vertical direction thatsubstantially linearly increase as approaching, from the installingpoint S, the center C of the width direction of the vehicle. The holediameters d₄, d₅, and d₆ here correspond to the vertical diameters ofthe middle lightening holes 62 a-62 c at centers of the respective holesof the width direction. Accordingly, the lines connecting the top endsand the bottom ends of the circles defining the respective holediameters in the vertical direction are lines each bending at the centerC of the width direction of the vehicle as one-dot broken lines show inFIG. 9. The second moment of area at a portion where in the middlelightening holes 62 a, 62 b, and 62 c are formed in the middle part 13of the underrun protection 10 decreases as approaching, from theinstalling point S, the center C of the width direction, and satisfiesthe requisite bending stiffness.

The second moment of area of the underrun protection 10 influences notonly the web 31 of the proximal panel 30 but also the flanges 32 and 33of the proximal panel 30, and the web 21 and the flanges 22 and 23 ofthe distal panel 20, and the reinforcing panels 40, 40. It is effectivethat the hole diameters of the lightening holes 61 a-61 c and 62 a-62 cof the vertical direction are set, considering the influence on thesecond moment of area on these elements of the underrun protection 10.

The hole diameters of the lightening holes 61 a-61 c and 62 a-62 c ofthe width direction of the vehicle do not much affect the bendingstiffness to compete against the horizontal loads P1, P2, and P3.However, external force to be applied to the underrun protection 10contains both a horizontal and a vertical components, and also even theapplication of the horizontal loads P1, P2, and P3 generates loads ofthe vertical components when the underrun protection 10 deforms.Considering the above, the hole diameters of the lightening holes 61a-61 c and 62 a-62 c in the width direction of the vehicle are alsolimited.

Considering three-dimensional stiffness to compete against a load havinga vertical component, the lightening holes 61 a-61 c and 62 a-62 c arenecessary to be omitted in vicinity of the bent portion serving asconnection between the web 31 and the flanges 32 and 33 of the proximalpanel 30. It is effective that the arrangement, the sizes, and theshapes of the lightening holes 61 a-61 c and 62 a-62 c are set, totallyconsidering the above.

In the illustrated example, the lightening holes 61 b, 61 c, and 62 bare set to be rectangular holes because a rectangular hole can moreeasily achieve the above requisite bending stiffness, can have a longerdistance from the bent portion connected to the flanges 22 and 23, andcan have a large opening area as compared with a circular hole. In theillustrated example, the corners of a rectangular hole are curved intoarc shapes to avoid concentration of stress thereon.

Comparing a circle hole with a rectangular (square) hole which holeshaving the same hole diameter in the vertical direction, the rectangularhole has a larger opening area than the circle hole. For the above, ifthe lightening holes 61-64 are formed into rectangular shape holes, theunderrun protection 10 can be lighter in weight as compared with circlelightening holes having the same vertical hole diameters as those of therectangular holes.

On the other hand, if the lightening holes 61-64 are formed into circleholes, the vertical lengths of the holes gently change along the widthdirection of the vehicle, so that the second moment of area along theextending direction of the underrun protection 10 is avoided fromsharply changing. For the above, the lightening holes 61-64 formed intocircles can prevent the strength or the stiffness of the underrunprotection 10 from locally lowering.

Next, the lightening holes 63 and 64 formed on the proximal panel 30 ofthe multiplexed panel part 80 will now be detailed.

As illustrated in FIG. 7, in each edge part 12, multiple (two in thisexample) lightening holes 63 are formed on the multiplexed panel part80, at which an extending part 42 of the reinforcing panel 40 that is tobe described below is in contact with the proximal panel 30. In themiddle part 13, multiple (four in total in this example) lighteningholes 64 are formed on the multiplexed panel parts 80 at which theextending parts 42 of the respective reinforcing panels 40 to bedetailed below is in contact with the proximal panel 30. Namely, twolightening holes 64 are formed on each of the multiplexed panel parts 80disposed at the left and the right in the middle part 13. The lighteningholes 63 and 64 are formed at a positions overlapping in thelongitudinal direction with the reinforcing panels 40, 40 thatconstitute the respective multiplexed panel parts 80.

As illustrated in FIG. 8 in each edge part 12, the lightening holes 63are formed inner side of the edge lightening hole 61 and are laterallyarranged. One lightening hole 63 a has a smaller hole diameter of thevertical direction than the other lightening hole 63 b formed closer tothe adjacent binder part 11 (i.e., on the inner side along the widthdirection of the vehicle). For the above, considering the proximal panel30 alone, a portion of the lightening hole 63 b closer to the binderpart 11 has smaller bending stiffness than that of the lightening hole63 a further from the binder part 11. However, since the portion of thelightening hole 63 b is reinforced by the wall part of the reinforcingpanel 40, the underrun protection 10 has higher bending stiffness at theportion of the lightening hole 63 b than the bending stiffness at theportion of the lightening hole 63 a in each edge part 12.

As illustrated in FIG. 9, among the four lightening holes 64 formed inthe middle part 13, two lightening holes 64 are laterally arranged ateach of the left and the right portions on the outer sides of the middlelightening holes 62. Focusing on two lightening holes 64 formed eitherside of the left and the right portions, one lightening hole 64 a has asmaller hole diameter of the vertical direction than the hole diameterof the vertical direction of the other lightening hole 64 b arrangedcloser to the adjacent binder part 11 (i.e., on the outer side along thewidth direction of the vehicle). For the above, considering the proximalpanel 30 alone, a portion of the lightening hole 64 b closer to thebinder part 11 has smaller bending stiffness than that of the lighteninghole 64 a further from the binder part 11. However, since the portion ofthe lightening hole 64 b is reinforced by the wall part of thereinforcing panel 40, the underrun protection 10 has higher bendingstiffness at the portion of the lightening hole 64 b than the bendingstiffness at the portion of the lightening hole 64 a in the middle part13.

After the above holes 61-65 are formed on the flat plate member, theplate is bent at the lines along the lateral direction to segment theweb 31 and the flanges 32 and 33, so that the proximal panel 30 ismanufactured. The proximal panel 30 is thereby manufactured such thatthe web 31 and the flanges 32 and 33 straightly extend along the lateraldirection, and the proximal panel 30 has a uniform cross section alongthe lateral direction.

[1-2-3. Reinforcing Panel]

As illustrated in FIGS. 7 and 10, each reinforcing panel 40 includes abody (main body of the reinforce panel) 41 having a section (the crosssection perpendicular to the lateral direction of the reinforcing panel)in the form of a channel and extending parts (extending parts of thereinforcing panel 40) 42 having cross sections forming straight linesand extending from the body 41 along the width direction of the vehicle.

The body 41 is arranged on each binder part 11 and has a section (i.e.,the cross section perpendicular to the lateral direction of reinforcingpanel 40) in the form of a C-shape, i.e., the channel shape.Accordingly, as shown in FIG. 10, the body 41 includes a web 41 avertically arranged and flanges 41 b and 41 c formed by bending theupper edge and the lower edge of the web 41 a in the same direction soas to protrude forward of the vehicle. Namely, the body 41 has the web41 a arranged on the proximal side and the flanges 41 b and 41 c havingtips directing the distal side.

The web 41 a extends along the width direction of the vehicle and isvertically arranged. On the web 41 a, installation holes 73 (thereference number is attached to one of the installation holes in FIG.10) as detailed below are formed.

The flanges 41 b and 41 c extend along the width direction of thevehicle and stick out along the longitudinal direction of the vehicle.The flanges 41 b and 41 c are configured in the same way except for thedifference whether each flange is connected at the upper edge or thelower edge of the web 41 a.

As shown in FIGS. 7 and 10, the extending parts 42 include a middleextending part 42 a that is arranged in a portion closer to the binderpart 11 (see FIG. 7) within the middle part 13 and an edge extendingpart 42 b arranged in a portion closer to the binder part 11 within theedge part 12 (see FIG. 7).

The lightening holes 71 and 72 are formed at the both edges of themiddle extending part 42 a and the edge extending part 42 b in the widthdirection of the vehicle, respectively. Here, one lightening hole 71 andone lightening hole 72 are formed on the middle extending part 42 a andthe edge extending part 42 b, respectively.

In cases where the proximal panel 30 omits thereon the lightening holes63 b and 64 b but does form thereon lightening holes 63 a and 64 a orwhere the proximal panel 30 omits thereon the lightening holes 63 a, 63b, 64 a, and 64 b, two lightening holes 71 and two lightening holes 72may be formed on each reinforcing panel 40 as illustrated by two-dotbroken lines in FIGS. 7 and 10. In this case, the lightening holes 71 aand 72 a closer to the both edges of the reinforcing panel 40 are formedto be larger than the respective lightening holes 71 b and 72 b closerto the center position of the reinforcing panel 40. Thereby, eachreinforcing panel 40 has higher bending stiffness at portions of thelightening holes 71 b and 72 b than the bending stiffness at portions ofthe lightening holes 71 a and 72 a.

As illustrated in FIG. 10, the middle extending part 42 a extends fromthe web 41 a of the body 41 to the inner side along the width directionof the vehicle. For the above, the middle extending part 42 a is formedso as to extend from the binder part 11 into the middle part 13 of theunderrun protection 10, as illustrated in FIG. 7. However, the middleextending part 42 a does not extend to reach the center C of the widthdirection of the vehicle in the middle part 13 of the underrunprotection 10.

If two lightening holes 71 a and 71 b are formed on the edge of themiddle extending part 42 a (here, the inner edge along the widthdirection of the vehicle), one of the lightening hole 71 a is formedlarger than the other lightening hole 71 b formed closer to the body 41of the reinforcing panel 40. This means that the lightening holes 71 aand 71 b arranged at the middle extending part 42 a are formed to becomelarger as approaching the edge of the middle extending part 42 a.

The edge extending part 42 b extends to the outer side of the web 41 aof the body 41 along the width direction of the vehicle. For the above,the edge extending part 42 b is formed so as to extend from the binderpart 11 into the edge part 12 of the underrun protection 10, asillustrated in FIG. 7. However, the edge extending part 42 b does notextend to reach the outer end of the edge part 12 of the underrunprotection 10 along the width direction of the vehicle.

As illustrated in FIG. 10, if two lightening holes 72 a and 72 b areformed on the edge of the edge extending part 42 b (here the outer edgealong the width direction of the vehicle), one lightening hole 72 a isformed larger than the other lightening hole 72 b formed closer to thebody 41 of the reinforcing panel 40. This means that the lighteningholes 72 a and 72 b are formed to be larger as approaching the end ofthe edge extending part 42 b.

[1-3. Arrangement of the Distal Panel, the Proximal Panel, and theReinforcing Panels]

Next, the relative arrangement of the distal panel 20, the proximalpanel 30, and the reinforcing panels 40 will now be described.

The common structure along the entire extending direction of theunderrun protection 10 will now be described with reference to FIGS. 6Aand 6B.

In the main body 10A of underrun protection 10, the distal panel 20 andthe proximal panel 30 are bound to each other so as to have parts(hereinafter called engaged overlapping parts) 91 (the reference numberis attached to its upper portion in FIG. 6) at which tips 22 a and 23 aof the flanges 22 and 23 of the distal panel 20 are brought into contactwith and bounded to tips 32 a and 33 a of the flanges 32 and 33 of theproximal panel 30, respectively, so that the main body 10A forms ahollow having a closed section. Here, description will now be made,assuming that the closed section that the underrun protection 10 and themain body 10A form is a rectangular box shape. The flanges 22 and 23 ofthe distal panel 20 are arranged outside the flanges 32 and 33 of theproximal panel 30, so that the distal panel 20 is fitted to the exteriorof the proximal panel 30.

The main body 10A of underrun protection 10 includes a front face 10 aformed of the web 21 of the distal panel 20, a rear face 10 b formed ofthe web 31 of the proximal panel 30, an upper face 10 c formed of theflange 22 of the distal panel 20 and the flange 32 of the proximal panel30, and a lower face 10 d formed of the flange 23 of the distal panel 20and the flange 33 of the proximal panel 30. The faces 10 a, 10 b, 10 c,and 10 d take shapes of flat plates.

In the illustrated example, the flanges 22 and 23 of the distal panel 20have shorter lengths along the longitudinal direction of the vehiclewithin the edge parts 12 and the middle part 13 (see FIGS. 1,2, and 7)of the underrun protection 10 as shown in FIG. 6B compared to the lengthof the flanges 22 and 23 of the distal panel 20 along the longitudinaldirection of the vehicle within the binder parts 11 (see FIGS. 1, 2, and7) of the underrun protection 10 as shown in FIG. 6A. The flanges 32 and33 of the proximal panel 30 are set to have shorter lengths along thelongitudinal direction of the vehicle within the binder parts 11 thanthe lengths of the flanges 32 and 33 of the proximal panel 30 along thelongitudinal direction of the vehicle within the edge parts 12 and themiddle part 13.

Next, description will now be made in relation to the structures of thebinder parts 11 (see FIGS. 1, 2, and 7) of the underrun protection 10with reference to FIG. 6A and in relation to the structures of the edgeparts 12 and the middle part 13 (for both, see FIGS. 1, 2, and 7) withreference to FIG. 6B.

Within each binder part 11 of the underrun protection 10, as illustratedin FIG. 6A, the reinforcing panel 40 is disposed so as to be in entirelycontact with the inner face of the rear face 10 b of the hollow space ofthe main body 10A, that is, so as to be entirely in contact with theinner face 30 a of the proximal panel 30. In other words, the entiretyof the reinforcing panel 40 is brought into contact with the inner face30 a of the proximal panel 30. Here, the wording “in entirely contact”and the relevant expressions include a state where the members (here,the proximal panel 30 and the reinforcing panel 40) do not keep incontact with each other because of manufacturing error of the membersand a state where, since at a portion that requires considerableaccuracy to bring the members into an contact with each other, the shapeof an inner member (here the reinforcing panel 40) is sometimes set suchthat the inner member is slightly apart from the outer member (here theproximal panel 30) to ensure the contact between the flat faces of themembers, and the members are not in contact with each other at such aportion.

Here, the body 41 of the reinforcing panel 40 is bound to the proximalpanel 30 such that the inner faces of a web 41 a and flanges 41 b and 41c are respectively in contact with the inner faces of the web 31 and theflanges 32 and 33. Specifically, the web 41 a of the body 41 of thereinforcing panel 40 is in contact with the web 31 of the proximal panel30 and the flanges 41 b and 41 c of the body 41 of the reinforcing panel40 are in contact with the flanges 32 and 33 of the proximal panel 30respectively.

Each reinforcing panel 40 is arranged so as to overlap the entirety ofthe corresponding binder part 11 of the underrun protection 10 along thelateral direction. A part at which the reinforcing panel 40 is arrangedis referred to as a reinforcing overlapping part 81 at which thereinforcing panel 40 overlaps the main body 10A. The multiplexed panelpart 80 (see FIG. 1) is arranged at the entirety of the binder part 11along the lateral direction and includes the reinforcing overlappingpart 81 in the cross section thereof.

In this example, the installation holes 73 (the reference number isattached to one of the holes in FIG. 6B, first installation hole, secondinstallation hole) are formed in the reinforcing overlapping part 81.Each installation hole 73 in the reinforcing overlapping part 81 is usedto fasten and fix the proximal panel 30 and the reinforcing panel 40together to the bracket 5 (see FIGS. 1 and 2) by means of a commonfastening member such as a common set of a bolt and a nut.

Here, the reinforcing panel 40 is bound to the inner face 30 a of theproximal panel 30 and also to the engaged overlapping part 91 so as toform a multiplexed part 85 at which three panels overlap and are incontact with each other. The multiplexed parts 85 include the flanges 41b and 41 c of the body 41 of the reinforcing panel 40; flanges 32 and 33of the proximal panel 30; and the flanges 22 and 23 of the distal panel20 that are respectively brought into contact in this order from theinside of the hollow. The lengths of the flanges 41 b and 41 c of thereinforcing panel 40; the flanges 32 and 33 of the proximal panel 30;and the flanges 22 and 23 of the distal panel 20 come to be larger alongthe longitudinal direction of the vehicle in this order.

The upper face (corresponding to the upper face 10 c of the main body10A) of the underrun protection 10 includes a single panel part 92formed only of the distal panel 20 at the distal side of the vehicle andan overlapping panel part 84 formed of multiple overlapping panels 20,30 and 40 being in contact with one another at the proximal side of thevehicle.

Likewise, the lower face (corresponding to the lower face 10 d of themain body 10A) of the underrun protection 10 includes a single panelpart 92 formed only of the distal panel 20 at the distal side of thevehicle and an overlapping panel part 84 formed of multiple overlappingpanels 20, 30 and 40 being in contact with one another at the proximalside of the vehicle.

The length L1 of the single panel part 92 along the longitudinaldirection of the vehicle is set to be almost the same as the length L2of the overlapping panel part 84 along the longitudinal direction of thevehicle (L1≈L2).

At the overlapping panel part 84, the reinforcing overlapping part 81 atwhich flange 32 of the proximal panel 30 is in contact with the flange41 b of the reinforcing panel 40 and the engaged overlapping part 91 atwhich the edge 22 a of the flange 22 of the distal panel 20 is incontact with the edge 32 a of the flange 32 of the proximal panel 30 arearranged so as to overlap in the longitudinal direction of the vehicle.The multiplexed part 85 is formed at a position where the reinforcingoverlapping part 81 overlaps the engaged overlapping part 91 along thelongitudinal direction of the vehicle within the overlapping panel part84.

In other words, the multiplexed parts 85 are formed by extending theflanges 22 and 23 of the distal panel 20 to the reinforcing overlappingparts 81 at which the flanges 41 b and 41 c of the body 41 of thereinforcing panel 40 respectively overlap the flanges 32 and 33 of theproximal panel 30 so as to overlap the reinforcing overlapping parts 81.

As shown in FIG. 6B, the reinforcing panel 40 is not arranged onportions (hereinafter called “non-installed portions”) of the edge parts12 and the middle part 13 of the underrun protection 10 except for theremaining portions adjacent to the binder parts 11. Namely, at thenon-installed portions of the edge parts 12 and the middle part 13 ofthe underrun protection 10, simplex body parts 90 formed only of themain body 10A of underrun protection 10 but not including thereinforcing overlapping part 81 are disposed.

As shown by the two-dot broken line in FIG. 6B, since the extendingparts 42 of the reinforcing panel 40 is arranged so as to extend fromthe binder part 11 to part of the edge part 12 and the middle part 13,the extending parts 42 are arranged at portions adjacent to the binderpart 11 in the edge part 12 and the middle part 13. Specifically, theextending part 42 is arranged so as to be in contact with the inner faceat the inside of the web 31 of the proximal panel 30. For the above, atportions adjacent to the binder part 11 in the edge part 12 and themiddle part 13, the reinforcing overlapping parts 81 at which thereinforcing panel 40 is in contact with the proximal panel 30 are formedand the multiplexed panel parts 80 are also arranged at the sameportions.

As shown by the two-dot broken line in FIG. 6B, within the middle part13 of the underrun protection 10, the above-described middle lighteningholes 51 (52) are formed on the flanges 22 and 23 of the distal panel 20which flanges 22 and 23 respectively form the upper face and the lowerface of the underrun protection 10.

These middle lightening holes 51 (52) are formed on a lightening holeforming parts 96, which are at the middle part of the underrunprotection 10 along the longitudinal direction of the vehicle andinclude an equidistance portion 95 which is equally distant from thefront face 10 a and from the rear face 10 b.

The edges 22 a and 23 a of the flanges 22 and 23 of the distal panel 20extend to positions that include the lightening hole forming parts 96where the middle lightening holes 51 (52) are formed. The inner faces ofthe edges 22 a and 23 a are brought into contact with the outer faces ofthe edges 32 a, 33 a of the flanges 32 and 33 of the proximal panel 30,respectively, and keeping this positional relationship, the edges 22 aand 23 a of the distal panel 20 are bound to the edges 32 a and 33 a ofthe proximal panel 30 by welding, respectively. The parts where theedges 22 a and 32 a are bounded and the edges 23 a and 33 a are boundedform the above-described engaged overlapping parts 91. In theillustrated example, lightening holes 51 (52) are formed on the singlepanel parts 92.

Also the edge lightening holes 53 (see FIG. 4) are formed on thelightening hole forming parts 96 including the equidistance portion 95in the underrun protection 10.

[2. Results and Effects]

The underrun protection 10 of the present invention, which has theabove-described structure, brings the following results and effects.

The reinforcing panels 40, 40 are arranged so as to be in contact withthe inner face on the side of the rear face 10 b of the hollow formed bythe main body 10A, so that the main part of the main body 10A (portionon the side of the rear face 10 b inside the hollow of the main body10A) is reinforced to ensure the strength and the stiffness of theunderrun protection 10. This structure can reduce the weight and theresultant material cost, concurrently ensuring the strength and thestiffness.

Specifically, since the distal panel 20, the proximal panel 30, and thereinforcing panels 40, 40, which collectively form the main body 10A,are made of ultrahigh tensile strength steel plates, the main body 10Athat ensures the strength and the stiffness but is light in weight canbe manufactured using thinner steel plates. Consequently, the cost forthe main body 10A can be saved as compared with using aluminum materialwhich is more expensive than steel.

In the underrun protection 10, there is a possibility that the binderpart 11 is affected by a large bending moment when a load is input intothe edge part 12 which has a cantilever structure. The edge parts 12 andthe middle part 13 require strength and stiffness smaller than thosethat the binder parts 11 require.

Since the simplex body parts 90, which is formed only of the main body10A of underrun protection 10 and which the reinforcing panel 40 doesnot overlap, are formed at the edge parts 12 and the middle part 13,which require relatively small strength and stiffness, the main body 10Aof underrun protection 10 can have simple structure that can contributeto weight reduction. In contrast, the multiplexed panel parts 80 havingthe reinforcing overlapping part 81 formed by bringing the reinforcingpanel 40 into contact with the entirety of the inner face of the rearface 10 b inside the hollow of the main body 10A are formed at eachbinder part 11, which requires relatively high strength and stiffness.Such a simple structure of the reinforcing panel 40 can increase thesecond moment of area, which can ensure requisite strength andstiffness.

Since arrangement of the multiplexed panel parts 80 and the simplex bodyparts 90 onto the parts (i.e., the binder parts 11, the edge parts 12,and the middle part 13) depends on the required strength and stiffnessthat the respective parts require, the total weight of the underrunprotection 10 can be saved, ensuring requisite strength and stiffness.

Since the closed section of the main body 10A of underrun protection 10has a rectangular closed section, which is a quite common shape of steelmaterial, the production cost for the main body 10A of underrunprotection 10 can be saved.

The main body 10A, which is formed by binding the distal panel 20arranged at the distal side of the vehicle and the proximal panel 30disposed at the proximal side of the vehicle to each other, can bemanufactured with ease, saving the production cost. For example, theabove underrun protection 10 equipped with the reinforcing panels 40 ismanufactured by firstly binding the reinforcing panels 40 to the innerface of the proximal panel 30 and then binding the distal panel 20 tothe proximal panel 30. Thereby, the reinforcing panels 40, 40 are easilyarranged inside the hollow of the underrun protection 10. Therectangular closed section of the main body 10A can be formed of thedistal panel 20 and the proximal panel 30 which are typical panelshaving channel shape section, so that the production cost of the mainbody 10A of the underrun protection 10 can be saved.

Each reinforcing panel 40 consists of the body 41 being arranged only onthe binder part 11 and being in contact with the inner face 30 a of theproximal panel 30; and the extending parts 42 extending from the web 41a of the body 41 along the width direction of the vehicle and being incontact with the inner face 30 a of the web 31 of the proximal panel 30.With this reinforcing panels 40, 40, predetermined portions (i.e., thebinder parts 11) and portions being adjacent to the binder parts 11 inthe edge parts 12 and the middle part 13) mainly including the binderparts 11 along the width direction of the vehicle are reinforced tosurely exert the strength and the stiffness of the underrun protection10 to compete against the bending moment.

The lightening holes 71 and 72 formed in the edges of the extendingparts 42 along the width direction of the vehicle can enhance the weightreduction. For example, if the lightening holes 71 a and 71 b formed inthe middle extending part 42 a are formed to be larger as approachingthe ends of the reinforcing panel 40 and/or the lightening holes 72 aand 72 b formed on the edge extending part 42 b are formed to be largeras approaching the ends of the reinforcing panel 40, the stiffness beingable to compete against the bending moment can be appropriately ensured,achieving weight reduction.

Since the distal panel 20 includes edge bents 29 formed at the bothedges of the lateral direction of the panel 20 and bent toward theproximal panel 30, the distal panel 20 can have the both edges of thelateral direction backward bent at the edge bents 29 so as to conform tothe backward-bent left and right edges of the front bumper to beattached at the immediately front of the underrun protection 10. Thisallows the proximal panel 30 to have a uniform cross sectional shapealong the lateral direction of the panel 30, so that processing cost ofthe proximal panel 30 can be saved.

In order to prevent the lightening holes from deforming when the panelsare subjected to the bending process, the lightening holes arepreferably formed at points which are not bent. The structure describedabove includes the proximal panel 30 that has a uniform cross sectionalshape along the lateral direction of the panel 30, so that the lightingholes 61-64 can be easily formed on the proximal panel 30.

Generally, if a member into which a load is input has larger supportingstiffness, buckling caused by the input load can be suppressed. Sinceeach engaged overlapping part 91 includes the multiplexed part 85 atwhich three panels 20, 30, and 40 overlap, the stiffness at themultiplexed part 85 and the peripheral thereof can be improved toinhibit generation of buckling.

Even when the binder part 11 is affected by a large bending momentcaused by, for example, inputting the load P1 into the underrunprotection 10, the improved stiffness at the multiplexed part 85 and theperipheral thereof can inhibit generation of buckling.

If the single panel part 92 has a longer length along the longitudinaldirection of the vehicle, a thinner plate thickness, and/or a lowerstiffness at the edges along the longitudinal direction of the vehicle,buckling is more easily generated when a load in the longitudinaldirection, such as the load P1, is input. Considering the above points,the presence of the overlapping panel part 84 formed by overlappingmultiple panels 20, 30, and 40 at the proximal side of the underrunprotection 10 can suppress the length of the single panel part 92 alongthe longitudinal direction, can also improve the stiffness at theproximal side of the underrun protection 10, and further can inhibitgeneration of buckling.

The length L1 of the single panel part 92 along the longitudinaldirection of the vehicle is set to be almost the same as the length L2of the overlapping panel part 84 along the longitudinal direction of thevehicle (L1≈L2). This setting to set the length L1 of the single panelpart 92 in the longitudinal direction to be short contributes toinhabitation of buckling.

Since the installation holes 73 are formed on the reinforcingoverlapping part 81 and each installation hole 73 on the reinforcingoverlapping part 81 is used to fasten and fix the proximal panel 30 andthe reinforcing panel 40 to the bracket 5 (see FIGS. 1 and 2) by meansof a common fastening member such as a common set of a bolt and a nut,the fastening member can be reduced as compared with cases where thereinforcing panel 40 is bound to the proximal panel 30 separately fromthe binding of the proximal panel 30 and the bracket 5. Consequently,such a common fastening member contributes to cost saving.

When the proximal panel 30 and the reinforcing panel 40 are fastened andfixed to the bracket 5 (see FIGS. 1 and 2) using a common fasteningmember such as a set of a bolt and a nut, the reinforcing panel 40functions as a washer for the bolt or the nut to improve the stabilityof fastening, reducing required elements.

The elements for fastening can be reduced as the above, which also leadsto weight reduction.

Since the lengths of the flanges 41 b and 41 c of the reinforcing panel40; the flanges 32 and 33 of the proximal panel 30; and the flanges 22and 23 of the distal panel 20 come to be longer along the longitudinaldirection of the vehicle in this order, the length of the flanges 41 band 41 c of the reinforcing panel 40 does not come to be longer, whichcontributes to weight reduction.

Since the panels 20, 30, and 40 are made of the same tensile strengthsteel plate (the same material) having the same thickness, it ispossible to eliminate the requirement to prepare multiple materials,reducing the material cost.

On the underrun protection 10, lightening holes formed on the upper faceand the lower face less reduce the stiffness than lightening holesformed on the front face and the rear face. Considering the above, sincethe multiple lightening holes 51, 52, and 53 are formed on the upperface and the lower face, the total weight of the underrun protection 10can be effectively suppressed, avoiding lowering the stiffness (i.e.,avoiding reduction in second moment of area).

Since the underrun protection 10 has lightening holes 51, 52, and 53being formed on the flanges 22 and 23 of the distal panel 20 and havingdifferent hole diameters of the lateral direction, the freedom inarrangement of the lightening holes 51, 52 and 53 along the extendingdirection of the underrun protection 10 can be improved as compared withcases where multiple lightening holes (hereinafter referred to as“equi-diameter lightening holes”) having the same hole diameter alongthe lateral direction are formed. For example, only the intervals ofsuch equi-diameter lightening holes can be changed while the lighteningholes 51, 52 and 53 of the present invention can change the diametersthereof along the lateral direction as well as the intervals. This meansthat the lightening holes 51, 52 and 53 can have an enhanced degree offreedom in arrangement on the underrun protection 10. Namely,determination of the region where lightening holes 51, 52 and 53 arearranged and the intervals of the lightening holes 51, 52 and 53 on theupper and lower faces of the underrun protection 10 has a high degree offreedom. This makes it possible to arrange lightening holes 51, 52 and53, considering various factors such as torsion acting on the underrunprotection 10 and the bending moment generated by a vertical load.Consequently, the weight of the underrun protection 10 can be furtherreduced.

The lightening holes 51, 52, and 53 are formed at the same points bothof the flanges 22 and 23 of the distal panel 20, which can reduce theprocessing cost and can easily eject foreign objects come into theunderrun protection 10.

The lightening holes 51, 52 and 53 formed on the lower face of theunderrun protection 10 allow foreign objects such as water and dirt tocome inside the underrun protection 10 to come out so that such objectsare not built up inside the underrun protection 10. This avoidscorrosion of the underrun protection 10 to enhance the durability of theunderrun protection 10.

Furthermore, the lightening holes 51, 52, and 53 formed on the lowerface of the underrun protection 10 allow the paint applied to underrunprotection 10 to easily drain out, which contributes to improvement inthe coating quality. In particular, when the underrun protection 10 iscoated by immersing into the paint, the paint can be surely drained outof the hollow through the lightening holes, so that the coating qualitycan be improved.

In the underrun protection 10, the higher stiffness of the corners ofthe rectangular section ensures the higher second moment of area whilethe stiffness of the middle portions of the respective faces definingthe rectangular section less affects the second moment of area.Considering the above, since the lightening holes 51, 52, and 53 areformed on the middle portion on the underrun protection 10 in thelongitudinal direction of the vehicle that less affects the secondmoment of area, specifically in the lightening hole forming part 96including the equidistance portion 95, which has an equidistance fromboth the front face 10 a and the rear face 10 b of the main body 10A ofunderrun protection 10, the weight of the underrun protection 10 can bereduced, efficiently preventing the decline of the second moment ofarea. Further, the lightening holes 51, 52, and 53, which are set tohave the same hole diameter in the longitudinal direction of thevehicle, reduce the processing cost thereof and prevent decline of thesecond moment of area.

Since the edges 22 a and 23 a of the flanges 22 and 23 of the distalpanel 20 extend to include the respective lightening hole forming parts96 where the lightening holes 51, 52, and 53 are formed, the lighteningholes 51, 52, and 53 are formed on the flanges 22 and 23 of the distalpanel 20. This structure makes it easy to form the lightening holes 51,52, and 53 both on the flanges 22 and 23 concurrently with press-cuttingof the outer shape of the distal panel 20.

The inner faces of the edges 22 a and 23 a of the flanges 22 and 23 ofthe distal panel 20 are respectively brought into contact with the outerfaces of the edges 32 a and 33 a of the flanges 32 and 33 of theproximal panel 30, and are bound to the outer face of the edges 32 a and33 a, respectively, by welding. This binding manner prevents the flanges32 and 33 of the proximal panel 30 projecting frontward of the vehicle,from exposing, which can further improve the safety.

The lightening holes 61 and 62, which are arranged on the proximal panel30 along the extending direction of the underrun protection 10, are setto have larger hole diameter in the vertical direction as departing fromthe binder parts 11, 11. This structure ensures large strength andstiffness by setting the lightening holes 61 and 62 adjacent to thebinder parts 11, 11 to have small hole diameters while efficientlylightens the weight of the underrun protection 10 by setting thelightening holes 61 and 62 apart from the binder parts 11, 11 to havelarge hole diameters. Consequently, the weight reduction of the underrunprotection 10 can be efficiently achieved, also ensuring the demandedstrength and stiffness.

Namely, multiple lightening holes 61 and 62 are formed so as to havelarger diameters in the vertical direction as departing from the binderparts 11, 11 to compete against the distribution of the bending momentof the underrun protection 10 caused by the load P1, so that therespective parts arranged extending direction of the underrun protection10 have bending stiffness that can compete against the distribution ofthe bending moment. Accordingly, the weight reduction of the underrunprotection 10 can be more efficiently achieved by forming the lighteningholes 61 and 62, ensuring the demanded bending stiffness to competeagainst the distribution of the bending moment caused by load P1.

Since the lightening holes 61 and 62 include the rectangular holes 61 b,61 c, and 62 b having rectangular openings when seen from the front,such large openings can promote efficient weight reduction.

Since the multiple lightening holes 61 formed on each edge part 12 areset to have larger hole diameters in the vertical direction as distantfarther from the binder part 11, underrun protection 10 can efficientlyachieve the weight reduction and ensure demanded strength and stiffnessin the edge part 12.

(Others)

The first embodiment is described as the above, but the presentinvention should by no means be limited to the foregoing embodiment.Various changes and modifications can be suggested without departingfrom the gist of the present invention. The respective structures of thefirst embodiment may be adapted, omitted, or combined according to therequirement.

The first embodiment assumes that the panels 20, 30, and 40 are made ofultrahigh tensile strength steel plates having a tensile strength of 980MPa. Alternatively, the panels 20, 30, and 40 may be made of variousultrahigh tensile strength steel plates having a tensile strength of 980MPa or more (e.g., 1.2 GPa or 1.5 GPa) or some of the panels may be madeof plate material that is different from ultrahigh tensile strengthsteel plates.

The above description assumes that the panels 20, 30, and 40 have athickness t of 3.2 mm. However, steel plates having any thickness can beused as far as the plates satisfy requisite strength and stiffness. Thepanels 20, 30, and 40 may of course have respectively different tensilestrengths and thicknesses from one another.

The above first embodiment described that the lightening holes 51, 52,and 53 on the upper face 10 c of the main body 10A of underrunprotection 10 are at the same positions of the lightening holes 51, 52,and 53 formed on the lower face 10 d. However, the lightening holes 51,52, and 53 on the upper face 10 c may not be formed at the samepositions as those on the lower face 10 d. Alternatively, the lighteningholes 51, 52, and 53 may be formed either one of the upper face 10 c orthe lower face 10 d. Further, as far as the lightening holes 51, 52 and53 are formed on the upper face 10 c and the lower face 10 d of the mainbody 10A of underrun protection 10, the lightening holes 51, 52, and 53may be formed outside the equidistance portions 95 and may not have thesame hole diameter of the longitudinal direction of the vehicle.

The first embodiment forms the lightening holes 51, 52 and 53 on theupper face 10 c and the lower face 10 d, the lighting holes 61, 62, 63and 64 on the rear face 10 b, and the lightening holes 71 and 72 on thereinforcing panels 40, 40. These lightening holes may partially formedand the remaining may be omitted.

The lightening holes 51, 52 and 53 may be formed on the flanges 32 and33 of the proximal panel 30, not on the flanges 22 and 23 of the distalpanel 20. In this alternative case, the flanges 32 and 33 of theproximal panel 30 are formed to extend to the lightening hole formingparts 96 (see FIG. 6B) that include the middle portion of thelongitudinal direction of the underrun protection 10 and also includethe equidistance portions 95 that are equally distant from the frontface and the rear face of the underrun protection 10.

The first embodiment assumes that the lightening holes 71 and 72 areformed on each reinforcing panel 40. Alternatively, some or all of theholes 71 and 72 may not be formed. Specifically, the lightening holes 71b and 72 b on a side of the center of the reinforcing panel 40 may notbe formed or all the holes 71 a, 71 b, 72 a, and 72 b may not be formed.

The flanges 41 b and 41 c of the body 41 of each reinforcing panel 40may have lengths of the longitudinal direction longer than the lengthsof the longitudinal direction that the flanges 32 and 33 of the proximalpanel 30 have. In this alternative example, the reinforcing panel 40 isbound to the inner face 30 a of the proximal panel 30, then bound to theinner faces of the flanges 22 and 23 of the distal panel 20, keeping incontact with each other. This structure arranges the overlapping panelparts 84 also at the distal side of the engaged overlapping parts 91 inaddition to the proximal side of the engaged overlapping parts 91.

Further, each reinforcing panel 40 assumes to have a extending part 42that extends from the body 41 along the width direction of the vehicle.Alternatively, either one or both extending parts 42 may not be formed.For example, the reinforcing panel 40 may be formed of the body 41 andeither one of the middle extending part 42 a or the edge extending part42 b and otherwise formed only of the body 41. Focusing only on thetechnique related to the lightening holes, the reinforcing panels 40, 40themselves may be omitted.

The first embodiment assumes the underrun protection 10 and main body10A have a rectangular closed section by binding distal panel 20 to theproximal panel 30 both having channel-shaped cross sections. However, asfar as the underrun protection 10 and the main body 10A have arectangular closed section, the cross sections of the panels 20 and 30should by no means be limited to a channel shape and various shapes ofthe cross sections can be suggested. For example, the cross sections ofthe panels 20 and 30 may be formed to be L shapes, or one of the crosssections of the panels 20 and 30 may be formed to be a channel shape andthe other may be formed to be a straight line. Likewise, the crosssections of the reinforcing panels 40, 40 may take various shapes.

The above first embodiment assumes each multiplexed part 85 is formed ofthree overlapping panels 20, 30, and 40. Alternatively, the multiplexedpart 85 may be formed of four or more overlapping panels by furtheroverlapping another panel with the inner face of the reinforcing panel40 or the outer face of the proximal panel 30. This structure improvesthe strength and the stiffness but increases the weight. Therefore, themultiplexed part 85 is preferably formed, considering compatibilitybetween the strength and the stiffness to be ensured and the weightincrease.

The first embodiment assumes the underrun protection 10 and the mainbody 10A form a closed rectangular section. However, the shape of thesection is not limited to a rectangle and may alternatively be a circleor a polygon except for a rectangular. In this alternative, thesectional shape of the panels 20, 30, and 40 conform to a closed crosssection that the underrun protection 10 and the main body 10A form.

Various known methods of binding can be applied to the bindings of thepanels 20, 30 and 40 of the underrun protection 10 with one another andthe bindings of the underrun protection 10 with the brackets 5, 5.Examples of the method of binding include welding bind such as plugwelding or fillet welding, or fastening with rivets, and blots and nuts.For the above, the installation holes 73 to bind the underrun protection10 with the brackets 5, 5 using a fastening member are not alwaysnecessary.

Furthermore, the main body 10A of underrun protection 10 is not limitedto being formed by binding the two panels of the distal panel 20 and theproximal panel 30 with each other and may alternatively be formed of asingle panel or three or more panels being bound. If the main body 10Ais formed of a single panel, various manners can be applied which areexemplified by binding the edges of the cross sections of the panels andextruding the panel.

The first embodiment assumes the entirety of the reinforcing panel 40 isbrought into contact with the inner face on the side of the rear face 10b in the hollow of the main body 10A. Alternatively, the reinforcingpanel 40 may be partially in contact with and partially overlap the mainbody 10A.

Into the underrun protection 10, loads may be input from variousdirections. Therefore, the underrun protection 10 has multipleprospective points that should ensure the strength and the stiffness,that is, points should be reinforced. In the underrun protection 10, areinforcing panel, which is brought into contact with the inner face ofthe proximal panel 30 to reinforce the rear face 10 b of the underrunprotection 10 in the first embodiment, may also be in contact with andreinforce at least one of the front face 10 a, the upper face 10 c, andthe lower face 10 d in the main body 10A of underrun protection 10. Inthis alternative, a point to be reinforced by the reinforcing panel ispreferably determined by considering assumable collision of the vehicleand installation state of the underrun protection 10.

For example, a reinforcing panel 40 may be bound to the inner face ofthe distal panel 40 (sic: 20), keeping in contact with each other. Inthis case, a single panel part formed only of the proximal panel 30 isformed on the proximal side of the vehicle and a simplex body partformed only of the main body 10A of underrun protection 10 but notincluding the reinforcing overlapping part 81 is formed on the proximalside. Meanwhile, an overlapping panel part where multiple panels such asthe distal panel 20 and the reinforcing panel 40 overlap is formed onthe distal side of the vehicle and the reinforcing overlapping partwhere the main body 10A overlaps the reinforcing panel 40 is formed onthe distal side of the vehicle.

The length L1 of the single panel part 92 in the longitudinal directionof the vehicle may be set shorter than the length L2 of the overlappingpanel part 84 in the longitudinal direction (i.e., L1<L2).

The first embodiment assumes that the underrun protection 10 isconnected to a pair of brackets 5, 5 disposed on the left and the right.Alternatively, multiple pairs of right and left brackets may be providedand connected thereto the underrun protection 10.

The above first embodiment assumes to have the lightening holes 61 and62 formed on the rear face 10 b of the main body 10A. The lighteningholes 61 and 62 are satisfactorily formed on at least one of the rearface 10 b and the front face 10 a. Further, the edge lightening holes 61may be omitted.

The hole diameters of the lightening holes 61 and 62 along the verticaldirection satisfactorily come to be larger as departing farther from theadjacent binder part 11, and the lines connecting the top and the bottomends of the respective holes may not be straight lines. For example, ifthe underrun protection has a curved distribution of the bending momentcaused by a load applied to the both edges of the extending direction ofthe underrun protection from the outside along the longitudinaldirection of the vehicle, the hole diameters of the respectivelightening holes may be set such that the lines connecting the top andthe bottom ends of the respective holes form curves.

The vertical diameters of the lightening holes 61 and 62 may be set tocompete against the distribution of the bending moment acting on theunderrun protection 10 and being caused not only by the load P1 but alsoby all the loads P1, P2, and P3, and otherwise may be set based on afactor different from the distribution of bending moment.

The shapes and the number of lightening holes 61 and 62 detailed aboveare examples and can be appropriately changed. As described above, thelightening holes 61 and 62 formed into rectangles enhance the weightreduction while the lightening holes 61 and 62 formed into circles avoidsharp change of second moment of area. Forming the lightening holes 61and 62 into the same shape can save the processing cost. An increase inthe number of lighting holes 61 and 62 by forming more lightening holes61 and 62 shorter in length of the lateral direction makes it possibleto more precisely adjust the bending stiffness along the extendingdirection of the underrun protection 10.

The edge lightening holes 61 b and 61 c and the second middle lighteninghole 62 b have the rectangular shapes when seen from the front, butalternatively may be formed to have squares seen from the front or maybe rectangles longer in lateral direction or longer in verticaldirection.

REFERENCE SIGNS LIST

-   1 frame-   2 side frame-   3 cross member-   5 bracket-   10 underrun protection-   10A main body of underrun protection-   10 a front face (distal face in the longitudinal direction of    vehicle)-   10 b rear face (proximal face in the longitudinal direction of    vehicle)-   10 c upper face-   10 d lower face-   11 binder part-   12 edge part-   13 middle part-   20 distal panel-   21 web-   22 flange-   22 a edge (edge part)-   23 flange-   23 a edge (edge part)-   29 edge bent-   30 proximal panel-   30 a inner face-   31 web-   32 flange-   32 a edge (edge part)-   33 flange-   33 a edge (edge part)-   40 reinforcing panel-   41 body (of reinforcing panel)-   41 a web-   41 b flange-   41 c flange-   42 extending part (of reinforcing panel)-   50 lightening hole (sic)-   51 circular middle lightening hole-   52 oval middle lightening hole-   53 edge lightening hole-   60 lightening hole (sic)-   61 edge lightening hole-   61 a first edge lightening hole-   61 b second edge lightening hole-   61 c third edge lightening hole-   62 middle lightening hole-   62 a first middle lightening hole-   62 b second middle lightening hole-   62 c third middle lightening hole-   63 a, 63 b lightening hole on multiplexed panel part-   64 a, 64 b lightening hole on multiplexed panel part-   65 connection hole-   71, 72 lightening hole-   73 installation hole (first installation hole, second installation    hole)-   80 multiplexed panel part-   81 reinforcing overlapping part-   84 overlapping panel part-   85 (tri-layered) multiplexed part-   90 simplex body part-   91 engaged overlapping part-   92 single panel part-   95 equidistance portion-   96 lightening hole forming part-   100 stiffener (sic)-   101 panel assembly (sic)-   111 base end (sic)-   112 tip (sic)-   112 a apex, flat face (sic)-   112 b, 112 c ridge line (sic)-   113 connection (sic)-   115 lightening hole (sic)-   102 predetermined clearance (sic)-   140 reinforcing panel-   141 a web-   L1 length of single panel part along longitudinal direction of    vehicle-   L2 length of overlapping panel part along longitudinal direction of    vehicle-   P1 load-   P2 load-   P3 load-   C center C of width direction-   S installing point S of bracket

The invention claimed is:
 1. A structure of an underrun protection boundto a plurality of brackets disposed at a front or a rear portion of aframe of a vehicle, comprising: a hollow having a closed section definedby a distal face in a longitudinal direction of the vehicle, a proximalface in the longitudinal direction, an upper face, and a lower face; anda plurality of lightening holes formed in a line along an extendingdirection of the underrun protection on at least one of the distal faceand the proximal face; wherein the plurality of lightening holes haverespective diameters in a vertical direction set larger as departingfarther from a binder part which is bound to one of the plurality ofbrackets.
 2. The structure according to claim 1, wherein: the pluralityof lightening holes have respective diameters in the vertical directionset larger as departing farther from the binder part, so as to competeagainst distribution of a bending moment on the underrun protection, thebending moment being caused by a load input to both edge portions in theextending direction of the underrun protection from a distal side of thelongitudinal direction.
 3. The structure according to claim 1, whereinthe plurality of lightening holes include at least one rectangular holehaving a rectangular front view.
 4. The structure according to claim 1,wherein the underrun protection includes a distal panel disposed on adistal side in the longitudinal direction and a proximal panel disposedon a proximal side in the longitudinal direction, wherein the distalpanel and the proximal panel are engaged with each other to form thehollow having the closed section.
 5. The structure according to claim 1,wherein: the distal face, the proximal face, the upper face, and thelower face each are a flat plate; and the closed section has arectangular shape.
 6. The structure according to claim 5, wherein: theplurality of lightening holes comprise edge lightening holes formed onthe proximal face and disposed in each edge part outside the binder partalong the extending direction of the underrun protection; and the edgelightening holes have respective diameters in the vertical direction setlarger as departing farther from the binder part toward an outer side inthe extending direction of the underrun protection.
 7. The structureaccording to claim 5, wherein: the underrun protection comprises adistal panel having a channel-shaped section and a proximal panel havinga channel-shaped or a straight line section bound together to form thehollow having the closed section; and the distal face is formed of a webof the distal panel, the proximal face is formed of a web of theproximal panel, and the upper face and the lower face are each formed offlanges of the distal panel or the proximal panel.
 8. The structureaccording to claim 7, wherein: the distal panel comprises edge bentsformed by bending the distal panel toward the proximal panel at bothedge portions in the extending direction; and the proximal panel isformed to have a uniform cross section along the extending direction.